1. Field of the Invention
The present invention relates to a field-effect transistor using nitride semiconductor, which is applicable to a power transistor for use in a power supply circuit of a consumer-electronics product such as a TV set and a method for fabricating the field-effect transistor.
2. Description of the Prior Art
Nitride semiconductor is wide gap semiconductor. For example, GaN (gallium nitride) and AlN (aluminum nitride) as examples of nitride semiconductor exhibit band gaps of 3.4 eV and 6.2 eV, respectively, at ambient temperature. An advantage of nitride semiconductor is that it has a larger insulation breakdown electric field and a greater electron saturation drift speed than those of compound semiconductor such as GaAs (gallium arsenide) or Si semiconductor. Nitride semiconductor has another advantage. That is, with nitride semiconductor, in a hetero structure of AlGaN/GaN formed on a substrate with a (0001) plane, electric charges are generated at a hetero interface due to spontaneous polarization and piezoelectric polarization and a sheet carrier concentration of 1×1013 cm−2 or more can be obtained even in an undoped state. Accordingly, by utilizing a two dimensional electron gas (2DEG) at a hetero interface, a hetero-junction field-effect transistor (HFET) with a large current density can be realized. With this as a background, power transistors using nitride semiconductor exhibiting advantages in increasing output and a breakdown voltage have been currently under active research and development.
FIG. 15 is a cross-sectional view of a known field-effect transistor using an AlGaN/GaN hetero structure. In the known field-effect transistor shown in FIG. 15, a low temperature GaN buffer layer 5502, an undoped GaN layer 5503 and an n-type AlGaN layer 5504 are formed in this order over a sapphire substrate 5501. A source electrode 5505 and a drain electrode 5506 are formed on the n-type AlGaN layer 5504. Each of the source electrode 5505 and the drain electrode 5506 is formed of a Ti layer and an Al layer on the n-type AlGaN layer 5504. A gate electrode 5507 is formed of a Ni layer, a Pt layer and an Au layer so as to be located between the source electrode 5505 and the drain electrode 5506. To provide isolation, part of the n-type AlGaN layer 5504 located in other part than a formation region is removed, for example, by dry etching. The field-effect transistor is a so-called normally ON type FET in which a drain current flows when a gate voltage is 0 V due to a high-concentration two dimensional electron gas generated at the hetero interface between the n-type AlGaN layer 5503 and the undoped GaN layer 5504.
However, when a GaN based HFET is applied to a power transistor, if the GaN based HFET is a normally ON type device, a safety problem might arise. For example, a circuit might be broken at the time of power failure, or the like event might be caused. Therefore, in order to make such a device be practically used as a power transistor, a GaN based HFET have to be a so-called normally OFF type FET in which a current does not flow with a gate voltage of 0 V applied. As a device structure which can satisfy the above-described requirements for known GaAs based compound semiconductor, a junction field-effect transistor (JEFT) using a pn junction as a gate has been proposed and also has been already in practical use (see J. K. Abrokwah et al., IEEE Transactions on Electron Devices, vol. 37, no. 6, pp. 1529-1531, 1990). In a JFET structure, a pn junction with a larger built-in potential than that of a Schottky junction is used as a gate and thus a gate turn-on voltage (a voltage at which a gate current starts flowing) can be increased and a gate leakage current can be reduced. In recent years, an example where adoption of the JFET structure for nitride semiconductor is studied has been reported (see L. Zhang et al., IEEE Transactions on Electron Devices, vol. 47, no. 3, pp. 507-511, 2000 and Japanese Laid-Open Publication No. 2004-273486).